With the advent of substantial quantities of oxygen available at reasonable cost there has been a revolution in the steelmaking industry based on the use of oxygen to decarburize and refine iron into steel. Indeed, the demise of the Bessemer converter and diminishing significance of the open hearth process attributable to the basic oxygen process is well known. Even where open hearth furnaces are still in use, many have been modified to utilize oxygen injection, most commonly through roof lances, to substantially increase capacity by reducing refining time by as much as 50 percent and more.
More recently, the bottom-blown basic oxygen process is beginning to receive considerable attention as being superior even to the conventional basic oxygen process. Whereas the conventional basic oxygen process utilizes a top lance to blow oxygen onto molten iron contained in an open tilting vessel, the bottom-blown basic oxygen process injects oxygen directly into the molten iron via tuyeres through the refractory vessel walls below the molten metal surface, for example, as disclosed in U.S. Pat. No. 3,706,549, Knuppel et al, issued Dec. 19, 1972. This new process is further characterized by utilizing double-pipe concentric tuyeres substantially flush with the inner refractory surface whereby oxygen is injected through a central orifice, and a protective jacket fluid, such as liquid or gaseous hydrocarbons, hydrogen, inert gases, carbon dioxide, steam, nitrogen or mixtures thereof, is blown through an annular orifice circumscribing the central oxygen orifice. The protective jacket fluid, usually natural gas, serves to envelop the oxygen upon initial injection into the melt to cool and protect the tuyere and the refractory walls adjacent to the tuyeres to thereby prevent rapid erosion of the tuyere and adjacent refractory.
The technical principles involved in the bottom-blown basic oxygen process are now also being incorporated into open hearth steelmaking facilities. That is to say, conventional open hearth furnaces are now being modified in some instances to provide double-pipe tuyeres through the refractory lining of the furnace below the molten metal surface, and then blowing oxygen, jacketed with a protective jacket fluid, into the molten metal. Indeed, many of the benefits of the bottom-blown basic oxygen process have been realized in using such modified open hearth furnaces. However, because the bottom of most open hearth furnaces is somewhat inaccessible, it has been necessary to locate the tuyeres through the furnace side wall. Even then, accessibility in some instances may be severely limited in view of pre-existing structural obstructions. Even in the absence of such obstructions, the installation of such tuyeres on existing furnaces requires a rather complicated drilling operation through the furnace side wall and complex external connections and appendages.
In addition to the above discussed minor problems, the placement of tuyeres in an open hearth side wall has caused a substantially more serious disadvantage as compared to conventional bottom tuyeres in tilting vessels. That is, the use of side wall tuyeres causes a rather rapid refractory erosion adjacent the tuyeres. Hence, despite the shielding gas envelope about the injected oxygen, bouyant forces acting on the injected oxygen and on the bath metal do result in gas and metal movement forces and patterns causing substantial refractory wear above and about the tuyeres. This erosion may be so severe that spot patching around each tuyere may be necessary after each heat. Furthermore, since the surrounding refractory is rapidly eroded, the tuyeres themselves are eroded substantially faster than experienced with bottom tuyeres in tilting vessels. This disadvantage is compounded by the fact that replacement of a plurality of tuyeres in a furnace side wall is not only difficult, but results in prolonged furnace downtime.